Hi & welcome! (and forum guidelines)

After years of repairing theremins and giving technical advice on this (and on other) forums, I thought that it might be a good idea to have a forum where I describe common problems and how to diagnose and/or fix them.

In order to keep this forum well structured, please [b]do NOT create new topics[/b] here. Let me do this. You are free, on the other side, to ask your questions [b]below[/b] my initial postings.

If you don't find your theremin problem here, this may be for several reasons:- I didn't have yet the time to write an article about this problem- Your problem is very specific and not a "usual case" on my workbench

In all these cases feel free to post your problems and/or questions in the common forums. I promise that I'll pick your posting up there.

I don't presently have a problem with my theremin. I just got my friend Allen to help me install your ESPE01 module last weekend...This was a bit dicey due to the fact that his bench was in his unheated garage, and we had to have the garage doors open in order to have enough light...and even with the doors open, there were several times where my heart was in my throat as I held the circuit board in my right hand and a flashlight in my left hand while he worked with a solder sucker and his soldering iron....Oh well, that's life in the wilderness of Middle Tennessee, USA.

Anyway, thank you ,thank you , thank you for your module...To my ear the tone is smoother and creamier, and the bass definition and separation are definitely there, though I now need to acquire some kind of subwoofer to do the module justice, since my small practice amp rolls off signifigantly somewhere just below 100hz.

To my mind, your module has transformed my humble Etherwave from an appliance into a real musical instrument...gratefully, jcn

Following the "Tuning the Volume Circuit" instruction it is impossible tochange VCA processor output voltage by means of turning slug of the L11. Voltmeter's readout at pin 12 of U3 (LM13600) is "-12V" constantly.For this reason the second part of the VCA has minimal gain andoutput signal at "Audio Out" is absent.

I thoroughly checked all parts and came to conclusion that the L11 variablecoil is out of order.

Where it is possible to order this part (in accordance with schematic it is47uH, manufacturer p/n TOKO 154ANS-T1019Z or RWRS-T1015Z)?

Thank you in advance.

P.S. It is strange that in accordance with TOKO datasheets this part number corresponds to 68uH.

1) it is rather improbable that a variable inductor is failing, as long as there was not a huge mechanical shock.

2) there were two different circuit layout variants. In both, the total inductance of the tank circuit is about 68uH. This is obtained in the older variant with a variable inductor of 68uH and in the newer variants (much more often) with a fixed 22uH inductor in series with a variable 47uH inductor.

3) Systematic diagnosing of the volume circuit consists of performing these steps systematically. That means do not go to the next step while the previous does is not accomplished.

- checking the volume oscillator without the volume loop connected and with an oscilloscope probe connected directly to the tank circuit. You should find there a sine wave of about 500kHz, 22Vpp, centered around 12V DC. If not, check all oscillator's components and eventually replace them until you get this. You'll need a "true" oscilloscope for that, a simple software solution which uses the sound card input of your computer will not work since the frequency is too high.

- checking the linearization coils for DC continuity: The 2.5mH coils should show a DC resistance of about 8 ohms, the 5mH about 15 ohms.

- setting up the theremin on a mic stand (since operating it on a workbench risks to mute the volume loop) and checking the DC level at the anode of D1 (take care to keep all cables away from the volume loop!) while changing the oscillator's frequency by tuning the variable inductor through the whole range and back. There must be a peak during that where the DC voltage changes by about 4V. If not, replace D1, even if it seems ok when you make a simple diode test with it.

- as soon as you are on this 4V peak in the tuning curve, let the variable inductor in that position and check with your hand near the volume loop if you can make voltage change back by approaching your hand to the loop. Either the volume circuit is working now or there is still a problem with the VCA pre-processor (1st half of the LM13600), but this would be a very rare phenomenon.

Don't hesitate to report your proceeding and progress back here so that I can help you in a more detailed way if you can't accomplish one of the above steps.

Thierry: I need to know - if I soldered a 125uH RF Choke used for Ham Radio Tube Transmitter Linear Amp Power Supply - on to the pitch antenna base inside the cabinet where the wire from the pc board is soldered to the fitting, would/could it damage the electronics in any way?

Explanation = EWS I just finished from the kit. Works fine on my original amp and diamond stage speaker that I built for my Theremax and B3 deluxe and Wavefront Classic, but I built a pair of amplified speakers for the B3 which works great with the B3, but with the EWS I can not get to "0 Beat" with the pitch knob adjustment.

I have no knowledge of electronics design --- so ---- after messing around with items of various masses of metal added to the antenna, I wrapped one of the leads of the choke around the wire going to the pitch fitting and the other lead is just laying next to the case. Now I can get to "0 beat" with either speaker/amp set ups.

Can I solder it permanently and not harm the Theremin?

In His Service --- Dana

PS. I didn't poke around with anything "on" the pc board, or try anything that involved adjusting stuff on the pc board. I know what some of the parts are called, but very little knowledge of what they do. :~)

Instead of adding additional capacitance to the pitch antenna by soldering whatever conductor which risks to limit the pitch range on the higher side, you should rather consider to retune slightly L6 which is the variable inductor in the middle of the circuit board. It has a graphite core with a hexagonal hole. The red plastic tool which is part of the EWS' accessories has to be used for that.

When you find that for example going clockwise with the pitch knob will make the situation better, but you arrive at the limits before the EWS is correctly tuned, turn L6 an eighth of a turn counterclockwise. Check if it is better and, if needed, repeat this step.

If the pitch knob is at the counterclockwise limit, turn L6 in a similar manner clockwise.

If there is a doubt, make rather smaller adjustments of L6 and repeat the procedure more often if needed. After each "tuning" turn put the cover back on the instrument (it influences the pitch field too) and let the oscillators warm up for 2 or 3 minutes until you try to tune it with the pitch knob in order to see if you are on the right way.

OK, I just tried the new (used) theremin and the volume knob seems defective to me, almost like a short or something. I cannot control the maximum volume, even with the knob turned all the way counter clockwise, it is very loud and it goes into a double whine with 2 pitches at the same time when I turn the knob at all. This also happens if I touch the volume antenna at all, then the noise stays there, whining with that double tone. If I turn it off and on it seems to reset itself, briefly, but still no volume control of maximum volume, then it flips over into that double tone again if I touch any of the controls. ARRRGH!

This theremin is not working properly. it this an easy fix or should I return it?